Vaccine and IgG experiments in certain inbred mice and in crossbreds have led to significant suppression of type C RNA viruses ecotropic viruses. As much as 4 to 5 logs of virus normally found in control can be suppressed in early life for as long as 40 to 50 days; relative to controls up to 2 logs of virus have been suppressed up to 200 days. Final tumor incidences in a number of experiments now on test will be available during FY 1977. Preliminary indications in several of the studies suggest that suppression of endogenous virus will result in significant protection against spontaneous cancers.